In a recent digital television set, since displayed information or operation items are increased in addition to the broadcast pictures, the number of operation buttons of a remote controller increases and operation becomes complicated.
In a view of this problem, a free-cursor type user interface has been proposed, in which an infrared-transparent filter that blocks a visible light and transmits a near-infrared light is provided at a lens part of a digital camera to take an image of a near-infrared light spot of the remote controller of the operator and to allow a selection of menu icons or the like on the display by moving a cursor on the display in conjunction with the movement of the operator's hand (see Patent Document 1, for example).
Recently, a digital camera is mounted to a display device including a display, such as a personal computer or a television set to enable so-called video communication in which a user has a conversation with a person apart using a display via a network such as the Internet or to add an energy saving function for monitoring a viewing condition of a viewer and turning off the power when the viewer is absent in front of the display for a long time.
When it is assumed that a free-cursor type interface of above described Patent Document 1 is applied to such display device and a single focus camera which does not include an optical zoom function but can reduce the cost is used as the infrared camera (hereinafter, simply referred to as a camera) for taking the infrared light spot of the remote controller, a camera imaging area becomes larger as a distance from the camera to the operator increases. In other words, the ratio of the imaging area of the camera and the display size changes according to the distance from the camera to the operator, and an arm stroke amount required when the operator holds the remote controller to operate the cursor changes.
The arm stroke amount of the operator is preferable to be constant regardless of the distance from the camera to the operator in a view of operability. It is thus proposed to change a ratio (magnification ratio) between an amount of displacement of the infrared light spot in the image taken by the camera and an amount of displacement of the cursor displayed on the display, according to the distance between the camera and the operator (see Patent Document 2, for example).
In this case, it is required to accurately obtain the distance between the camera and the operator; however, in Patent Document 2, the distance between the camera and the operator (remote controller) is obtained by using that an intensity of an infrared light of the remote controller changes inversely proportional to the square of the distance.